Telephone companies provide telephony service to their customers using systems known generically as either circuit or packet switched systems. Circuit switched systems send data signals continuously over telephone lines to provide both Internet access and conventional analog telephony or simply plain old telephone service (“POTS”). Packet switched systems, such as the Internet, serve as a transport medium for carrying voice and data in a data packet form. One of the main differences between these technologies is that the circuit switched systems have a continuous circuit whereas packet switched systems transmit bursts of packets and are therefore discontinuous.
Currently, there is a growing trend to transition from use of circuit switched networks to packet switched networks. For example, the voice over Internet protocol (VoIP), is becoming a popular method for making a telephone call over a packet switched network. The International Telecommunications Union (ITU) standard H.323 is a current standard for transmitting voice over the Internet. Session Initiation Protocol (“SIP”) is another popular protocol. In these networks, voice may be compressed by standard means such as G.723.1 or G.729.A and used for transmitting compressed speech.
Unfortunately, however, signaling within a packet switched network can have more stringent constraints than within a circuit switched network to limit bandwidth consumption. In one instance, when modems communicate through a packet network, the modems exchange control signals during the communication. The packet switched network endpoint that communicates with a given modem will receive the control signal and send it through another endpoint that then communicates the signal to the other modem. However, the packet network endpoint may have to wait long periods of time to detect or identify certain signals received from the modem. For example during modem answer sequences, typical standards recommend and/or require waiting a specified amount of time to detect certain signals before proceeding to the next sequence (e.g., an answer tone (like “ANS”) detection is typically detected for 1 second by a modem before a training sequence can begin). Also, some signals are sufficiently complex and require significant amounts of time to simply generate all the information contained in the signal (e.g., an ANSam signal includes a 180 degree phase reversal every 450 ms). Other transmission standards as set forth in RFC 2833, the full disclosure of which is herein incorporated by reference, also impose limitations for communicating signals within and between the network endpoints.
Additionally, during the time that a packet network endpoint waits to identify the signal, vocoders in the endpoint may destroy the incoming signal. Care should be taken to ensure that the FAX or Voice Band Data terminations at the circuit-switched endpoints can function properly even with these known limitations of packet networks. For example, when transmitting these signals inband, over a lossy network with a lossy compressed codec, signal information is destroyed. This should be minimized or avoided altogether during the answer phase and training phase of a voice band modem or FAX transmission. The packet network should not cause the modulating endpoint to misidentify the signal sent from another modulating endpoint.